1. Technical Field
The present disclosure relates to micro-light-emitting diodes (micro-LEDs).
2. Description of Related Art
Electronic devices increasingly include display screens as part of the user interface of the device. As may be appreciated, display screens may be employed in a wide array of devices, including notebook computers and handheld devices, as well as various consumer products, such as smart phones and tablet personal computers. Some of those devices result in the screen display being used under a variety of environmental conditions. One example is having a display screen mounted within an automobile. A driver's or passenger's ability to view the screen while traveling in a vehicle is affected by the outside light conditions, for example. Different levels of screen brightness are required during daytime hours as compared to nighttime hours. Accordingly, there is a need for an efficient and relatively simple way of adjusting the brightness of a display screen in response to ambient light conditions.
In the recent years, light-emitting diodes (LEDs) have become popular in general and commercial lighting applications. Accordingly, since the screen displays are used under a variety of environmental conditions, high ambient light conditions or low ambient light conditions, a wider dynamic range of the LEDs brightness output becomes important in display screens.
However, in the current density versus voltage (J-V) characteristics of an LED, current density is approximately an exponential function of voltage near the threshold, so a small voltage change may result in a large change in current density. Further, the I-V characteristics is determined by the following equation I:
                    I        =                              I            0                    ⁡                      (                                          exp                ⁡                                  (                                                            q                      ⁡                                              (                                                  V                          -                                                      Ir                            s                                                                          )                                                              nKT                                    )                                            -              1                        )                                              equation        ⁢                                  ⁢        I            where I is the current through the LED, I0 is the maximum current for a large reverse bias voltage (reverse saturation current), q is the electron charge, V is the voltage across the diode, rs is the series resistance, k is Boltzmann's constant, and T is the absolute temperature.
In addition, one disadvantage of the conventional LEDs is that the LEDs have a wide range of operating current density, and the J-V curve of the LEDs is nonlinear as the current density is too low or too high. When the current density is too low, the forward voltage of each of the LEDs is different from others due to the difference produced by the manufacture process, and hence the brightness uniformity of the LEDs is difficult to control. When the current density is too high, the conversion efficiency of the LEDs is low due to the thermal issue.
Furthermore, if the voltage is below the threshold or on-voltage no current will flow and the result is an unlit LED. If the current density is too high the current will go above the maximum rating, the result is overheating and potentially destroying the LED. Therefore, the LED brightness is difficult to be controlled linearly due to the above reason.